Missing docs for TWITTER_API_SERVICE and CONTENTPORT_IDENTITY_KEY (self-hosting blocker)

[olololoe110399]

I found two environment variables that appear required for core features, but they are not documented in the repository setup:

TWITTER_API_SERVICE
CONTENTPORT_IDENTITY_KEY
These are referenced in backend routes for tweet indexing and feed refresh, but there is no clear explanation of:

what service TWITTER_API_SERVICE should point to
how CONTENTPORT_IDENTITY_KEY is issued/validated